The present invention relates to an apparatus and a method for determining an overtravel time of a machine having a moveable machine part and an optoelectronic, camera-based protection device designed to stop any movement of the machine part.
There are numerous machines and machine installations having moving machine parts whose movement is hazardous for personnel in the vicinity of the machine. Typical examples include presses, machine tools, robots or transport and conveyor installations. It is known to protect the hazardous working area of such machines by means of optoelectronic protection devices. A light barrier or a light grid is often arranged in front of the hazardous working area. When light beams of the light barrier or light grid are interrupted, a stop signal is produced, by means of which the dangerous movement is stopped, or the entire machine is even shut down. In addition to conventional light barriers and light grids, optoelectronic protection devices have been proposed for some years, where a “virtual” protection zone in front of the hazardous working area is achieved by suitable evaluation of camera images. (In this case, in the following text and for the purposes of the present invention, the term “protection zone” is used, for the sake of simplicity, covering both one-dimensional linear protection areas, two-dimensional surface protection areas and three-protection protection volumes). By way of example, EP 0 904 402 A1 discloses a safety camera by means of which the entry area of a lift door is monitored.
Regardless of the nature of the optoelectronic protection device, the protection zone must be arranged sufficiently far away from the moving machine part so that a person or some other object which enters the protection zone cannot reach the moving machine before the machine part has actually come to rest. Since mass inertia, signal processing times and other delay times prevent a moving machine part to be brought to rest at the very same time that a protection zone is entered, an overtravel time of the machine part after triggering of a stop command is an important safety-relevant parameter for successful safeguarding of the machine. The overtravel time may vary because of ageing or because of different loading or fitting of a machine. Reaction times of a machine control system may vary because of individual programming For this reason, it is necessary to reliably determine the actual overtravel time of a machine, that is to say the time difference between the generation of a stop signal and the hazardous movement coming completely to rest.
DE 290 06 578 U1 discloses a system which is intended to allow accurate measurement of an overtravel time of a machine with an optoelectronic protection device. In this case, the optoelectronic protection device is a light grid with a plurality of light beams, which is arranged in front of a press. When light beams are interrupted, the light grid produces a stop signal which is transmitted to a control unit of the press. A reflector is arranged on the moving machine part, which in this case is the press stamp, and this reflector is moved together with the press stamp. A (further) optoelectronic sensor device having a plurality of light-sensitive sensor elements is arranged opposite the reflector. The reflector is illuminated by a light beam. The position of the reflector can therefore be recorded by means of the sensor unit. The overtravel time of the press stamp is determined by means of a storage oscilloscope, which produces a curve representing the position of the reflector, plotted against time. The stop signal for stopping the press stamp can be produced in various ways using the reflector. It is proposed that the protection zone of the light grid be interrupted by means of a so-called engagement element, which can be operated by an evaluation and control unit via a magnet. Furthermore, the stop signal can be produced by a light barrier which emits a signal to the storage oscilloscope when the reflector is detected and initiates the process of stopping the press stamp. Finally, a switching relay of the machine control system, by means of which the press is switched off, can be monitored by means of a magnetic sensor, in order to start the measurement of the overtravel time.
A mechanical engagement element by means of which light beams of a light grid may be automatically interrupted in order to trigger an overtravel measurement of a machine part is described in a product leaflet issued by German company HHB GmbH, 82211 Herrsching, Germany. The engagement element, which is referred to as an “auto hand” in the leaflet, can also be used to automatically operate emergency-off pushbuttons or foot-operated switches.
DE 10 2006 058 707 A1 discloses an apparatus and a method for determining an overtravel time and an overtravel distance of a machine, wherein the start of the measurement is initiated by operating an emergency-stop device.
DE 10 2004 058 472 A1 discloses an optoelectronic protection device for a press or a similar machine, wherein at least two optical barriers run in front of the moving press stamp at different distances in front of the moving press stamp. The first optical barrier triggers a stop signal as soon as it is interrupted. The second optical barrier, which is arranged closer to the moving press stamp, can be used to check if the overtravel distance of the moving press stamp is less than the relative distance between the two optical barriers in the movement direction.